Block copolymers have been developed rapidly within the recent past, the starting monomers usually being monoalkenyl arenes such as styrene or alpha-methylstyrene and conjugated dienes such as butadiene and isoprene. A typical block copolymer of this type is represented by the structure polystyrene-polybutadiene-polystyrene (SBS). When the monoalkenyl arene blocks comprise less than about 55% by weight of the block copolymer, the product is essentially elastomeric. Moreover, due to the peculiar set of physical properties of such a block copolymer it can be referred to more properly as a thermoplastic elastomer. By this is meant a polymer which in the melt state is processable in ordinary thermoplastic processing equipment but in the solid state behaves like a chemically vulcanized rubber without chemical vulcanization having been effected. Polymers of this type are highly useful in that the vulcanization step is eliminated and, contrary to vulcanized scrap rubbers, the scrap from the processing of thermoplastic elastomers can be recycled for further use.
Those block polymers which comprise in part conjugated diene polymer blocks have one substantial shortcoming, namely, their susceptibility to oxidation or ozonolysis. Substantial improvement both in stability and compatibility with alpha-olefin polymers have been made by hydrogenation of such block polymers. The hydrogenation may be non-selective, selective or complete. Certain technical advantages have been found for selective hydrogenation wherein at least about 80% of the aliphatic double bonds are reduced and no more than about 25% of the aromatic double bonds are reduced by hydrogenation. Block copolymers having selectively hydrogenated conjugated diene blocks are disclosed in U.S. Pat. No. 3,595,942. These selectively hydrogenated block copolymers are repesented by the structure poly(monoalkenyl arene)/hydrogenated polydiene/poly(monoalkenyl arene). A typical block copolymer of this type is polystyrne/poly(ethylene/butylene)/polystyrene obtained by selectively hydrogenating polystyrene/polybutadiene/polystyrene (SBS) where the polybutadiene has a relatively high 1,2-cis configuration (35-55 mol percent).
While these selectively hydrogenated block copolymers have vastly improved stability over their unsaturated precursors, they have certain shortcomings which it would be desirable to eliminate or minimize. Chief among these is poor processability. It is possible, of course, to improve processability by diluting the polymer with extending oils and the like. This normally results in a drastic reduction in other physical properties, particularly, heat resistance, tensile strength and properties associated therewith. Blends of these block copolymers with a second resin for processability improvement are known, but in most instances the second resin is a relatively nonpolar polymer. Highly polar polymers typically are not compatible with these block copolymers. Blends of polystyrene/polybutadiene/polystyrene block copolymers (SBS) with nylon polymers have been disclosed in U.S. Pat. No. 3,546,319 (15% SBS rubber in polyamide), Belgium Pat. No. 703,498 (35% nylon in SBS), and Japan Pat. No. 7,138,611 (5 to 50% SBS in polyamide). The SBS differs from the selectively hydrogenated SEBS in that the solubility parameter, in units of (cal/cm.sup.3).sub.1/2 as calculated by Small's method (J. Applied Chemistry, Vol 3, page 71, 1953) of the polybutadiene specific block is 8.4 rather that of the poly(ethylene/butylene) block is only 7.9. Because of the significant difference in these block solubility parameters, SBS and SEBS have profoundly different melt viscosity -- shear stress behavior. Furthermore, the solubility parameters of highly polar polymers is much greater than that of either the B or EB blocks (for example the solubility parameter of nylon 66 is 13.6). Therefore, the blending characteristics of non-hydrogenated and hydrogenated block polymers are highly different, and, particularly when blended with highly polar polymers, such as polyamides, are unpredictable.